The invention relates to the technical field of hot or cold forging to form hollow parts made of light alloys. The invention also relates to the foundry technology sector in which the process of casting material around a core pre-positioned and held in a mould is well known.
The invention relates to the application of the process to all technical fields to form parts with high mechanical properties and particularly for some parts or components used in the car and cycle industry but not restrictively.
The technology associated with forging hollow parts is well known. This includes, for example, the forging of ingots of pre-drilled material, and also the radial forging of forging blanks or ingots requiring time and a plurality of forging stages.
The forging of hollow parts with core pins put in place to form recesses is also known. This technique requires the use of core pin insertion mechanisms and an automated system capable of operating at the stroke rate of power hammers for good productivity. Apart from the fact that the shapes of these recesses must not be too complex as they generate the shape of the core pins, said recesses are always exposed to allow the core pins to be put in place, and create use restrictions. Additionally, the kinematics calculation is not always easy to implement.
The method for machining the parts is also known but it is costly in terms of machining time and in initial material to be machined.
The process is also known of forging hollow parts using two half-parts of complementary shape which are welded to each other along their peripheral edge provided for said assembly. This requires sophisticated and costly high-frequency welding apparatus and a quality control of the welding in consideration of the external loads placed on the part so formed. This technique requires each of the half-parts to be formed independently and then assembled as indicated above.
Furthermore, combining processes for the foundry casting of materials of the light alloy type followed by a forging phase is also well known through the many patents of the Applicant who originated them with EP patent 119 365 and the developments thereof EP 1250204, EP 1219367. The manufacture of hollow parts by forging and casting has a great and ever-increasing number of applications.
Apart from the patents of the Applicant, a proposal has been made to include a core in the forged part using a hybrid process combining casting and forging. This is disclosed for example in patent EP 850825 which is specific to a cycle pedal crank. The solution described in this document, although interesting, has some drawbacks or limitations. Prior to the forging operation, the foundry core-prints are removed, and during forging, the core in not in a stable situation. There is also the likelihood of core waste produced during the forging operation causing surface defects.
Patent EP 850825 makes no allowance for the possibilities of the core being deformed when striking during forging. Said patent is restricted to cycle pedal cranks in which the constraints are quite different from those relating to the technical components used in the design of motor vehicles.
A process for manufacturing hollow forgings as defined in patent PCT WO 2009/050382 is also known. The document illustrates and describes the use of a core. However, the process described has a great number of drawbacks. It sets out to seal the blank so that the core is completely isolated from the external environment. This seal is provided by blocking each gas vent used to position the core with a blocking element. Each gas vent is itself blocked by a stiffening element in the form of a rod or metal pin.
According to this patent and as indicated in the description the core-prints associated with the core are positioned in the zones to be machined. To remove the core and the aforementioned associated vents, the process described in the patent requires the part obtained to be drilled and pre-machined in order to knock out the core. This is a very heavy constraint and tricky to implement. The operator must also fettle the final part. Fettling involves recovering miscellaneous and incompatible materials such as aluminium and steel, the aluminium being contained in the forged burr and the steel being the material constituting the sealing means (core pins, added core-prints). This requires sorting in the event of recycling.
Thus in the two patents EP 850 825 and WO 2009/050382 the operations described are costly in both economic and environmental terms.
The process described in this patent presupposes that the part is hollow right through, which restricts the use thereof. In fact, the specifications issued by car manufacturers with whom the applicant works have no requirement for the parts so worked to be hollow in the totality of their volume. The technique described in the patent PCT WO 2009/050382 does not allow for parts to be formed that have both hollow zones and solid zones.
Both aforementioned patents thus have limitations inherent in the processes employed.